Two proteins, barnase, the extracellular ribonuclease of Bacillus amyloliquefaciens, and barstar, its intracellular inhibitor, are used as a model system for the study of protein folding a protein-protein interactions. Barnase is one of an homologous group of ribonucleases occurring in both prokaryotes and eukaryotes. Recombinant DNA techniques are being applied to the project with three major aims; 1) to facilitate production, 2) to examine the structural and control sequences of the genes and 3) to tailor specifically designed modifications in the sequences to test theories of protein folding. The complete barnase gene is lethal when reassembled in either E. coli or B. subtilis. With directed mutation of the essential histidine-102 the gene is expressed in both organisms, but the mutant products are not correctly processed. A vector has been devised, based on the promoter and signal sequence of the of the Pho-A gene, which allows facile production of the muture (mutant) enzyme in high yield (16-20 mg/liter. The barstar gene has also been cloned in E. coli, where it is expressed as a specific inhibitor of barnase.